1. Field of the Invention
The invention relates generally to coalbed methane collection from horizontal boreholes drilled in coal seams, and more specifically to perforation systems that punch gas collection holes in plastic liner pipes in such boreholes only if there is no water infiltration of the adjacent media surrounding a particular pipe section.
2. Description of the Prior Art
Natural deposits of coal are sometimes charged with methane gas, e.g., coalbed methane (CBM). Such CBM can be sold at a profit if the costs to produce it are minimized. In the past, such CBM was simply dissipated and wasted because it was too expensive to collect it. One of the expenses in coal mines has been dealing with groundwater and the problems caused when it floods the mine workings.
Horizontal drilling methods are used to dig CBM-gas collection boreholes in virgin coal deposits. If the drilling encounters groundwater, e.g., as caused by faulting in the coal deposit or the punching through of the floor or ceiling into water-laden sandstone, such water can flood the whole borehole and prevent CBM-gas collection. Such water can either be pumped out, sealed out, or avoided by smart drilling.
In U.S. Pat. No. 3,934,649, issued Jan. 27, 1976, Pasini and Overby describe a method for removing methane from coalbeds prior to mining the coal. It comprises drilling at least one borehole from the surface into the coalbed. The borehole is started at a slant rather than directly vertically, and as it descends, a gradual curve is followed until horizontal and the desired portion of the coalbed is intersected. Approaching the coalbed in this manner and fracturing the coalbed in the major natural fraction direction cause release of large amounts of the trapped methane gas.
U.S. Pat. No. 4,299,295, issued Nov. 10, 1981, to Amzi Gossard, describes a process for drilling spaced horizontal boreholes in coal and other mineral deposits. The boreholes are typically in excess of 1500 feet long, and patterned to maximize gas removal. Directional drilling guidance is provided by a continuous downhole survey tool connected to data display devices by an internal drill rod cable system. Directional drilling control is provided by a positive displacement motor positioned at the end of the drill string and operated by a flow of drilling fluid through the drill string from the drilling rig. The mineral strata surrounding the borehole is periodically hydrofractured to permit effective removal of the gas. The hydrofractionation is effected without removal of the drill string or survey instruments from the borehole. Upon completion of the borehole, the drill string is removed and gas which enters the borehole from the surrounding deposit is withdrawn. David Summers, et al., describe in U.S. Pat. No. 4,317,492, a method and apparatus for drilling horizontal holes in geological structures from a vertical bore. The geological structures intended to be penetrated in this fashion are coal seams, as for in situ gasification or methane drainage, or in oil-bearing strata for increasing the flow rate from a pre-existing well. Other possible uses for this device might be for use in the leaching of uranium ore from underground deposits or for introducing horizontal channels for water and steam injections.
The present inventor, Larry G. Stolarczyk, has described methods and equipment for imaging coal formations in geologic structures in many United States Patents. Some of those Patents are listed in Table I, and are incorporated herein by reference.
TABLE IU.S. Pat. No.IssuedTitleUS04577153Mar. 18, 1986Continuous Wave Medium FrequencySignal Transmission Survey ProcedureFor Imaging Structure In Coal SeamsUS04691166Sep. 01, 1987Electromagnetic Instruments ForImaging Structure In GeologicFormationsUS04742305May 03, 1988Method For Constructing VerticalImages Of Anomalies In GeologicalFormationsUS04753484Jun. 28, 1988Method For Remote Control Of A CoalShearerUS04777652Oct. 11, 1988Radio Communication Systems ForUnderground MinesUS04879755Nov. 07, 1989Medium Frequency Mine Communica-tion SystemUS04968978Nov. 06, 1990Long Range Multiple Point WirelessControl And Monitoring SystemUS04994747Feb. 19, 1991Method And Apparatus For DetectingUnderground Electrically ConductiveObjectsUS05066917Nov. 19, 1991Long Feature Vertical Or HorizontalElectrical Conductor DetectionMethodology Using Phase CoherentElectromagnetic InstrumentationUS05072172Dec. 10, 1991Method And Apparatus For MeasuringThe Thickness Of A Layer Of GeologicMaterial Using A Microstrip AntennaUS05087099Feb. 11, 1992Long Range Multiple Point WirelessControl And Monitoring SystemUS05093929Mar. 03, 1992Medium Frequency Mine Communica-tion SystemUS05121971Jun. 16, 1992Method Of Measuring Uncut Coal RibThickness In A MineUS05146611Sep. 08, 1992Mine Communication Cable AndMethod For UseUS05181934Jan. 26, 1993Method For Automatically AdjustingThe Cutting Drum Position Of A Re-source Cutting MachineUS05188426Feb. 23, 1993Method For Controlling The ThicknessOf A Layer Of Material In A SeamUS05260660Nov. 09, 1993Method For Calibrating A DownholeReceiver Used In ElectromagneticInstrumentation For Detecting AnUnderground ConductorUS05268683Dec. 07, 1993Method Of Transmitting Data From ADrillheadUS05301082Apr. 05, 1994Current Limiter CircuitUS05408182Apr. 18, 1995Facility And Method For The Detec-tion And Monitoring Of PlumesBelow A Waste Containment Site WithRadiowave Tomography ScatteringMethodsUS05474261Dec. 12, 1995Ice Detection Apparatus ForTransportation SafetyUS05686841Nov. 11, 1997Apparatus And Method For The Detec-tion And Measurement Of LiquidWater And Ice Layers On The SurfacesOf Solid MaterialsUS05769503Jun. 23, 1998Method And Apparatus For A RotatingCutting Drum Or Arm Mounted WithPaired Opposite Circular PolarityAntennas And Resonant MicrostripPatch Transceiver For Measuring Coal,Trona And Potash Layers Forward,Side And Around A Continuous MiningMachineUSRE032563Dec. 15, 1987Continuous Wave Medium FrequencySignal Transmission Survey ProcedureFor Imaging Structure In Coal SeamsUSRE033458Nov. 27, 1990Method For Constructing VerticalImages Of Anomalies In GeologicalFormations
The present inventor, Larry Stolarezyk, describes the drilling, imaging, and coal-bed methane production ahead of mining in U.S. patent application, Ser. No. 09/905,552, filed Jul. 12, 2001, and now U.S. Pat. No. 6,497,457, and is incorporated herein by reference. Such directional drilling can be used to bore several horizontal shafts through a coal deposit with its natural overburden still intact. Any methane gas permeating the coal deposit is pumped out and preferably sold as natural gas to commercial and residential customers, or used locally in support of mining operations. The methane gas evacuation continues until the concentrations are reduced to safe levels for mining. But before mining begins, ground penetrating radar equipment is lowered into the boreholes for electronic imaging studies of the coal deposit. One borehole is used for a transmitter and another for a receiver. Many measurements are made at a variety of frequencies and equipment positions within the boreholes. Such studies estimate the electrical conductivity of the surrounding material, and thereby give clues where and how much coal is actually deposited. More accurate assessments of the coal reserve can then be developed from this information, and the results can be used with much higher confidence than the traditional reserve estimates obtained with conventional methods. The better numbers are preferably used to dictate business planning.